Eye contact is looking someone in the eyes who, in return, is looking into your eyes.
If someone holds your gaze a second longer than usual you may assume the other person feels attracted to you. Eye contact is vital to flirting, and especially powerful if it is coupled to a warm smile.
If someone makes eye contact and keeps looking at you it’s a compliment. If you are not interested, accept the compliment but don’t look back. If the attraction is mutual and you want more you can return the stare and eventually approach this person, starting a conversation by some small talk.
It’s hard to know whether eye contact is part of a game or an invitation. Some flirt for fun, others because they are looking for a significant other. If you are interested you can always start a conversation to see if it can progress into a friendship or relationship.

Games people play

See also: flirting and mirroring